What are the short, easy-to-memorize dhikrs suitable for day and night and all occasions?

There are absolute remembrances (adhkar) that are prescribed at all times, including:

La ilaha illa Allah (There is no god but Allah): This is the best form of remembrance. Subhan Allah (Glory be to Allah), Al-hamdu lillah (Praise be to Allah), and Allahu Akbar (Allah is the Greatest): These are charities, the most beloved words to Allah, and more beloved to the soul than anything upon which the sun rises. La hawla wa la quwwata illa billah (There is no might or power except with Allah): This is a treasure from the treasures of Paradise. Subhan Allah al-azim wa bihamdihi (Glory be to Allah, the Most Magnificent, and with His praise): A palm tree is planted in Paradise for the one who says it. Subhan Allah wa bihamdihi, Subhan Allah al-azim (Glory be to Allah, and with His praise; Glory be to Allah, the Most Magnificent): Two phrases light on the tongue, heavy on the scale, beloved to the Most Merciful. Salawat (sending blessings) upon the Prophet (peace and blessings be upon him): For every blessing one sends upon the Prophet, Allah sends ten blessings upon that person. Istighfar (seeking forgiveness): The Prophet (peace and blessings be upon him) used to seek forgiveness more than seventy times, or a hundred times, a day.

There are also remembrances that are restricted to specific times or situations, such as morning and evening remembrances, those before sleep, and those after prayers. These remembrances can be found in books such as Al-Adhkar by An-Nawawi, Sahih al-Kalim at-Tayyib by Al-Albani, and Hisn al-Muslim by Al-Qahtani.